Conventionally, the voltage converting circuits can have their applications in solar energy systems, being in parallel with the household electricity, so that they have a wide range of DC voltage at the input terminals thereof. For household usage, the solar cell modules can be connected in parallel or in series to each other to supply the home appliances with electricity of proper DC voltages. However, throughput of the solar energy system can be affected once one of the solar cell modules was out of order for some how, so a DC-to-AC voltage converter was included in a solar cell module. Low-voltage power generation of the solar cell module can be transferred to an AC power by the DC-to-AC voltage converter and then in paralleled with the household electricity; thereby, reliability and efficiency of the household power supply system can be improved.
The DC-to-AC voltage converter in solar cell modules can be constructed by a DC-to-DC voltage converting circuit of high-frequency switching in the first stage and a DC-to-AC voltage converting circuit of low-frequency switching in the second stage. With regard to the DC-to-DC voltage converting circuit therein, it is based on the pulse-width-modulation (PWM) mechanism. The PWM technique to operate the DC-to-DC voltage converting circuit is of slow response and not in the soft-switching condition for all possible loading scenarios. Therefore, it is in need to develop an operating method for controlling the active switches in the DC-to-DC voltage converting circuit to improve its performance.